I tried posting this before the crash, so I'll try to re-construct my thoughts.
When I was in high school, we had 55 minute periods except for "fourth hour" which actually consisted of four 25 minute segments (with 5 minute pass periods in between each).
So, your fourth hour class would meet for 4AB or 4BC or 4CD. Then one of the segments was "homeroom" in the same classroom as the two segment class, and the final segment was the actual lunch time.
Under a plan like that, I could see making science that needed a 1-1/2 period be fourth hour only, and ditching the 'homeroom' time for added lab time.
Even so, I took both biology and chemistry (in different years) and never once did our labs require more consecutive time than one class period (some were multi-day by design, though).